Technical Field
The present invention relates to a control circuit for a switching power supply device and to a switching power supply device, and particularly relates to a control circuit for a switching power supply device and to a switching power supply device that reduce the occurrence of noise by introducing jitter (frequency spreading) to a switching frequency.
Background Art
A switching power supply device rectifies and smoothes a commercial AC voltage and switches the smoothed voltage using a switching element in order to generate a prescribed DC voltage and output the voltage to a load. When the switching element is switched at a prescribed switching frequency, higher-order harmonics that take the prescribed switching frequency as their fundamental waves are produced at the same time. These higher-order harmonics radiate outside the switching power supply device as radiated EMI (electromagnetic interference) noise and conducted EMI noise. Such EMI noise negatively affects the operations of other electronic devices, and thus standards set limits ensuring that no more than a set amount of EMI noise is produced. Radiated EMI noise, which is radiated outside the switching power supply device as radio waves, is limited to a measured frequency range of 30 MHz to 1 GHz. Meanwhile, conducted EMI noise, which escapes to the exterior from a power supply cord of the switching power supply device connected to a commercial AC voltage, is limited to a measured frequency range of 150 kHz to 30 MHz.
With respect to current conducted EMI standards, there is discussion, in the field of power electronics, of expanding the measured frequency range of EMI noise to lower frequencies than 150 kHz to suppress conducted EMI noise even in lower measured frequency ranges. Specifically, setting the lower limit frequency of the measured frequency range to 9 kHz is being considered.
If the measured frequency range is expanded, new conducted EMI noise reduction measures are necessary in that 9 kHz to 150 kHz measured frequency range. In other words, if the switching frequency is, for instance, the typically-used 65 kHz, the fundamental wave and the second-order harmonic thereof (130 kHz) are also subject to measurement. Moreover, while the energy of the switching frequency is greatest at the fundamental wave thereof and decreases progressively at the second- and third-order harmonics, the fundamental wave and the second-order harmonic, where the noise energy is high, are newly subject to measurement. This means that stronger measures for noise reduction are necessary compared to current standards, in which noise reduction measures only need to be taken from the third-order harmonic of the switching frequency, which has an even lower energy than the second-order harmonic.
Typical measures for reducing such conducted EMI noise in a switching power supply device include providing an EMI filter at a location where an AC voltage is received, and taking circuitry-related measures within a power supply IC (Integrated Circuit) that controls the switching power supply device.
An EMI filter is configured by combining inductor and capacitor components. In an EMI filter, the components have higher constants as the frequency to be suppressed decreases, and conversely have lower constants as the frequency to be suppressed increases. Expanding the measured frequency range to frequencies lower than those in current standards increases the constants of the components. This means that the components will be larger, which makes it difficult to fit the components into the spaces available in current EMI filters, increases costs, and so on. As such, implementing measures for reducing conducted EMI noise in the 9 kHz to 150 kHz measured frequency range on the power supply IC side eliminates the need for such measures on the EMI filter side; and even if EMI filter-side measures are necessary, the scale of such measures can be kept to a minimum.
As opposed to measures for reducing conducted EMI noise using an EMI filter, a technique known as spread spectrum clocking or spread spectrum clocking is known as a measure for reducing conducted EMI noise using a power supply IC (see Patent Document 1, for example). This introduces jitter (frequency spreading) to the switching frequency and spreads the noise so as not to be concentrated at a specific frequency. Although the amount of power radiated itself does not change, the peak level of the spectrum is reduced, which reduces the noise energy.
FIG. 6 is a diagram illustrating an example of a spectral distribution before and after switching frequency modulation. In FIG. 6, the horizontal axis represents frequency and the vertical axis represents a reduction amount. In this diagram, the reduction amount is indicated as an amplitude of 0 dB (decibels) before modulation.
It can be seen from FIG. 6 that a pre-modulation spectrum 101, which is represented by the broken line, is present in a narrow frequency band concentrated at a central frequency of a high-frequency component. When this spectrum 101 is modulated through spread spectrum clocking, the spectrum is spread throughout a given frequency band, resulting in the spectrum 102 represented by the solid line. In the example illustrated here, the spectrum 102 has a peak 12 dB lower than the peak of the spectrum 101, and thus it can be seen that the spread spectrum clocking technique provides a conducted EMI noise reduction effect of at least 12 dB.
Although the spectrum 102 illustrated here uses a sine wave as the modulating signal for determining the modulating frequency, note that other waveforms aside from sine waves are sometimes used. In actuality, a triangular wave is typically used as the modulating frequency signal in switching power supply devices, and although the spectral distribution obtained in the case where a triangular waveform is used does differ from the spectrum 102 illustrated in FIG. 6, it is generally a similar spectral distribution.
In other words, in the spectral distribution obtained by using a triangular waveform for the modulating frequency, protruding areas 103 are formed by the amplitude near both ends overshooting the amplitude near the center, in the same manner as in the spectral distribution of the spectrum 102. Such protruding areas 103 hamper the conducted EMI noise reduction effects, and thus the conducted EMI noise reduction effects can be further increased by eliminating such protruding areas 103.
A technique for eliminating the stated protruding areas appearing near both ends of a frequency-spread spectrum by employing a waveform having a special shape as the waveform of the modulating frequency is known (see Non-Patent Document 1, for example).
FIGS. 7A and 7B are diagrams illustrating the waveform of a modulating signal (a modulating waveform) and a spectrum in this frequency spreading technique, where FIG. 7A indicates an optimal modulating waveform and FIG. 7B indicates an example of a spectral distribution obtained by modulating the clock frequency with the optimal modulating waveform.
The optimal modulating waveform indicated in FIG. 7A is a waveform having a curve referred to as a “Hershey's Kiss” (registered trademark). This Hershey's Kiss waveform has a special shape that cannot be formed easily, and thus in the field of signal circuits, this waveform is formed using a dedicated IC known as an SSCG (Spread Spectrum Clock Generator).
Using this Hershey's Kiss waveform as a clock modulating waveform eliminates overshoot near both sides of the spectrum and smoothes the spectrum, as indicated in FIG. 7B. As such, a greater effect of reducing conducted EMI noise can be achieved through the frequency spreading technique, and thus the same conducted EMI noise reduction effect can be achieved even when the technique is applied in a switching power supply device.